Engine combustion using diesel fuel may generate particulate matter (PM) (such as soot and aerosols) that can be exhausted to the atmosphere. To enable emissions compliance, diesel particulate filters (DPFs) may be included in the engine exhaust, to filter out exhaust PMs before releasing the exhaust. In addition, one or more soot sensors may be used to diagnose the DPFs and such soot sensors may be coupled upstream and/or downstream of the DPF.
As such, various types of soot sensors have been developed to sense soot production and release. One example approach shown by Paterson in U.S. Pat. No. 8,310,249 discloses soot sensors that collect particulate matter on charged electrodes. The soot sensor comprises opposed electrodes separated by an insulator with a gap in between to prevent current flow. When soot particles start to accumulate on the sensor, a bridge is created between the electrodes allowing for current to flow. The change in current is used as an indication for soot deposition.
However, the inventors herein have recognized potential disadvantages with the above approach. As one example, non-uniform or low soot deposit on the surface can occur due to biased flow distribution across the sensor surface, resulting in inaccurate voltage and current readings across the gap. Additionally, it may be difficult to reach sensor regeneration temperatures due to large flow impingement on the surface in some sensor designs. Further still, the sensor may become contaminated due to impingement of large diesel particulates or water droplets on the surface of sensor electrodes. Contamination may also be caused by the large diesel particulates or water droplets infiltrating into the inner protection tube of the sensor.
In addition to electrode-based sensors, pressure-based soot sensors have also been developed. For example, as described by Sun et al. in U.S. Pat. No. 8,209,962, differential pressure across a particulate filter may be used for monitoring filter performance. Therein, when the differential pressure is less than a threshold, a leak in the particulate filter may be determined. However, this method may also suffer from interference from large aggregated particulates or water droplets impinging on the sensor.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method includes: diverting exhaust gas from downstream of a first filter into each of parallel first and second pathways, the second pathway including a second filter coupled to an electric circuit; and indicating degradation of the first filter based on an interval between successive regenerations of the second filter. In this way, DPF diagnostics may be performed with higher accuracy and reliability without the results being corrupted by flow and soot loading distribution or impingement of droplets.
As an example, exhaust gas may be diverted from a main exhaust pipe, downstream of a DPF, into two parallel conduits (a first and a second exhaust pathway) outside of the main exhaust pipe via an inlet pipe. The inlet pipe may include perforations that allow water droplets and aggregated particulates to be trapped and released into the tailpipe. The second exhaust pathway may be fitted with a metallic particle filter (MPF) and an electric circuit may be coupled to the filter. Further, the two pathways may merge at a location downstream of the filter, wherefrom exhaust is returned to the main exhaust pipe. As exhaust gas diverted from the main exhaust pipe is received in the two parallel pathways, exhaust PMs, such as soot, may be deposited on the MPF of the second pathway, while exhaust containing soot flows unobstructed through the first pathway. As a result a pressure and/or temperature difference is generated, which is measured by a pressure or temperature sensor coupled to the two pathways. Once the pressure or temperature difference reaches a threshold, the electric circuit coy pled to the MPF is closed to initiate regeneration of the filter. Regeneration completion is indicated based on a drop in the pressure or temperature difference. Further, a time interval elapsed between successive regenerations is learned. As such, if the DPF becomes degraded (such as due to age or durability issues), an increasing amount of soot may escape from the DPF, and travel onto the metal filter. As a result, the metal filter may have to be cleaned more frequently. Thus, based on a decrease in the time interval elapsed between successive regenerations of the metal filter in the second exhaust pathway, degradation of an upstream DPF may be determined, and appropriate actions may be taken.
In this way, by diverting a portion of exhaust gas from an exhaust pipe to a soot sensor with a metal filter, located downstream of a diesel particulate filter, degradation of a particulate filter can be detected based on an amount of soot leaking from the particulate filter onto the metal filter. The technical effect of trapping soot particles on the metal filter selectively included in one of the two pathways, is that a differential pressure or differential temperature between the pathways can be advantageously used to learn the soot load of the metal filter. The technical effect of trapping aggregated particulates and water droplets in an inlet pipe of the soot sensor, and redirecting them to the exhaust tailpipe, is that impingement of aggregated particulates and water droplets on the soot sensor is reduced, allowing for more accurate and reliable soot detection. By relying on a time interval between successive regenerations of the metal filter to detect DPF degradation, is the diagnostics may be rendered more sensitive and less affected by variations in soot loading distribution on the metal filter. Overall, accuracy and reliability of soot sensing and diagnosing of an exhaust particulate filter is increased, enabling higher emissions compliance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.